1. Field of the Invention
The present invention relates to a brake disc material used for a brake assembly of trucks and other large-sized transport vehicles.
2. Description of the Related Art
Brake assemblies for automobiles may be generally classified as drum brake assemblies or disc brake assemblies. Disc brake assemblies are mainly used for small-sized automobiles and drum brake assemblies for large-sized automobiles. However, drum brake assemblies cannot be used effectively for large-sized trucks with a loading capacity of 8 tons or more, since the brake action would not be uniform for every brake or wheel when braking from a high speed. Under extreme conditions, such one-sided action of the brakes would result in accidents.
In a disc brake assembly, a rotary member (below, the "disc") mounted on a wheel is pressed against a pad to create the brake action. The action to the pad is relatively small in a small-sized automobile, therefore there is no particularly serious friction heat and friction wear.
The load applied to the disc is higher, however, in a large-sized automobile, such as a large-sized truck. The friction heat generated by the braking is therefore high. A maze of cracks is thus often generated in discs made of conventional material.
Specifically, since the disc is subjected to great force and hence considerable friction heat when braking a large-sized truck or the like, the temperature of the disc rises quickly. Such a quick temperature rise occurs with every braking action. The disc is therefore exposed to repeated heat impact. This results in the formation of the maze of cracks in a conventional cast-iron disc when used over a long period of time.
The temperature rise results in a local difference in the braking force on the surface of a disc and also promotion of disc wear. If the cracked disc is continued to be used, the cracks propagate further and, eventually, the disc is destroyed.
Because of the above, there is much difficulty in the maintenance of disc brakes in large-sized trucks.
Japanese Examined Patent Publication (Kokoku) No. 58-53713 discloses a disc material containing a small amount of chromium of from 0.6 percent to less than 1.2 percent and a large amount of nickel of from 1.5 percent to 3.0 percent. The disc referred to in this publication is a hat type where only the crack propagation need be prevented and no wear resistance need be considered. The addition of the disclosed alloying chromium and nickel aims to enhance the fracture toughness. The disclosed disc material is, however, inappropriate for use as the rotary disc of a large-sized automobile, such as a large-sized truck, where a considerably high load is applied to the disc.